The present invention relates to a vehicle-mounted ignitor in which an insulated gate type semiconductor device is used as a main switching element, and in particular to a vehicle-mounted ignitor, an insulated gate type semiconductor device and an engine system having a current restricting capability.
Enhancement in performance of the ignitors for engines has been strongly demanded for the energy-saving in vehicles. The ignitor is adapted to fire the fuel in the engine by generating a high voltage such as several tens thousand volts from a low voltage battery mounted on a vehicle depending upon the rotational number of the engine for discharging ignition plugs.
A vehicle-mounted ignitor in which an insulated gate type semiconductor device is used as a main switching element is disclosed in JP-A-09-280147.
It has been known that in the ignitor including a current restricting circuit for suppressing an overcurrent, an oscillation may occur when the current restricting current begins to operate, which causes various problems such as occurrence of noise and damages of devices. In order to prevent these problems from occurring, JP-A-09-280147 provides a current supply circuit which supplies a gate with a current from a collector for suppressing an abrupt increase in collector voltage at the beginning of the current restriction by supplying the gate with a current from a collector. A detailed example of the current supply circuit in which a high voltage constant current element which is combined with an IGBT and MOSFET is used is illustrated in FIGS. 1, 8 and 9 of JP-A-09-280147. The current flowing from the collector to the gate is restricted to a constant value by using the saturation characteristics of IGBT and MOSFET. Use of resistors and capacitors is illustrated in FIG. 7 of JP-A-09-280147.
The saturated current of the IGBT and MOSFET which are used to form the current supply circuit in JP-A-09-280147 largely varies with their temperatures. It tends to decrease as the temperature increases. There are certainly variations in saturation characteristics among IGBTs and MOSFETs. The variations in the characteristics of the current supply circuits cause the restricted current of the main insulated gate type semiconductor device (the invention will be described with reference to an IGBT) to vary so that the current capacity of the entire of the ignitor circuit should be designed to be larger. If it is assumed that the variation be 2 amperes in case in which the restricted current is 10 amperes, at least the allowable current capacity of the circuit should be designed to be 2 amperes or more. Accordingly, the capacity of the used circuit components (for example, capacitors, resisters) and the diameter of the cross-section of the wires becomes larger, which leads to an increase in the bulk and weight of the ignitor. This invites an increase in size of the engine and the fuel consumption. In order to increase the current capacity, it is necessary to increase the cross-section of the wire of the ignition coil, which increases the coil size. Since ignition coils are inserted into the engine body in a so-called distributorless ignition system in which one ignition coil is disposed for each cylinder of the engine, the increase in the size of the coils will invite an increase in size of the engine block. Further, holes of the engine block into which the coils are adapted will become larger in size so that the strength of the engine block will be lowered. As a result, the durability of the engine will be lowered.
Although the above-mentioned problem in which the characteristics vary will not occur in case in which the current supply circuit consists of resistors and capacitors, a problem will occur in that IGBT may malfunction. For example, when the IGBT is turned off to ignite the ignition coil, a high voltage which is about 400 V is applied across the collector and emitter of the IGBT in a usual ignitor. However, the gate voltage will increase due to a current flowing to the gate from the collector in the above-mentioned circuit. The IGBT is turned on again, for restricting the voltage across the collector and the emitter. This will suppress the inherent features of the ignitor. The voltage across the secondary coil of the ignition coil will decrease so that no arc is generated in the ignition plug. If the resistance is increased to prevent this, supply of the current to restrict the oscillation will become insufficient so that the oscillation suppression will be lowered.
It is an object of the present invention to provide a vehicle-mounted ignitor, insulated gate type semiconductor device and engine system having a current restricting capability which is compact in size and less in capacity, and causes no oscillation.
In an aspect of the present invention, an insulated gate type semiconductor device such as IGBT and power MOSFET is used as main switching element and an oscillation suppress current supply circuit potentially connects a main terminal of the main switching element having a higher potential to a control terminal of said switching element without interposing other semiconductor switching element in a vehicle-mounted ignitor including said oscillation suppress current supply circuit.
A phrase xe2x80x9cpotentially connectedxe2x80x9d used herein means xe2x80x9cto provide a function to make both terminals equally potential by connecting via a resistor and a diode for supplying a current. For example, a capacitor does not have such function and is thus omitted. In a preferred embodiment, said current supply circuit comprises a resistor and a diode which are connected in series with each other and this circuit is connected between one of main terminals having higher potential and said control terminal.
This provides a vehicle-mounted ignitor having a capability of restricting a current and causing no oscillation without using any semiconductor switching element which is liable to have variations in temperature characteristics.
In another aspect of present invention, a vehicle-mounted ignitor including an oscillation suppress current supply circuit is characterized in that a circuit is provided which bypasses said current supply circuit to one of a pair of main terminals having a higher potential when a signal for driving the insulated gate type semiconductor element is not input to the control terminal. It is preferable to provide a bypass switching element which is connected between said current supply circuit and one of said pair of main terminals having a lower potential; and a circuit which turns on the switching element when a signal for driving the main semiconductor element is not input to the control terminal.
In the other aspect of the present invention, a vehicle-mounted ignitor including an oscillation suppress current supply circuit is characterized in that said current supply circuit is configured to supply the control terminal with current from one of the main terminals of the main insulated gate type semiconductor element, which is higher in potential, and to restrict said current to a predetermined value.
In a further aspect of the present invention, the vehicle-mounted ignitor including an oscillation suppress current supply circuit is characterized in that said current supply circuit supplies a current to said control terminal from said main terminal having a higher potential when said voltage of said main terminal having a higher potential is lower than a predetermined value and restricts the supply of the current to said control terminal when the voltage of said main terminal is higher than the predetermined value. In a preferred embodiment, said current supply circuit comprises a series circuit of two resistors which are connected in a direction toward said control electrode from said main terminal having a higher potential; and a constant voltage element for restricting the potential on the series connection to a predetermined value.
In a further aspect of the present invention, an insulated gate semiconductor device including a control electrode which is formed on the main surface of the semiconductor substrate in such a manner that an insulated film is interposed therebetween is characterized in that said insulated film is configured to have a partially thick structure.
In a further aspect of the present invention, a vehicle-mounted ignitor comprising a primary coil of an ignition coil and an insulated gate type semiconductor device which are connected in series with a direct current source; an ignition plug connected to a secondary coil of the ignition coil, to which a higher voltage generated across the secondary coil by switching of said semiconductor device is applied; a current restricting circuit for restricting a main current flowing through said semiconductor device to a predetermined value or less by controlling the potential on a control electrode of said semiconductor device; and a current supply circuit for supplying said control electrode with a current from one of a pair of main terminals of said semiconductor device having a higher potential, is characterized in that said current supply circuit is configured to connect said one of said main terminals having a higher potential to said control electrode via not switching element, but a resistor, said system further including an ignition coil unit comprising said semiconductor element, said current restricting circuit and said current supply circuit which are incorporated as a chip; a connecting terminal of said ignition plug which is provided at one end of the ignition coil unit; and an ignition plug which is connected to said connecting terminal, said ignition coil unit and ignition plug being integrated with each other and being embedded in the engine wall so that said ignition plug is exposed within a combustion chamber.